1. Field of the Invention
The present invention generally relates to a structure and a method for overlay measurement and, more particularly, to a structure and a method for overlay measurement capable of analyzing material layers and the overlay error between the material layers.
2. Description of the Prior Art
In semiconductor manufacturing, overlay measurement is for determining the alignment between a printed layer and a previous printed layer. It is crucial to achieve precise and accurate alignment for every element so that the quality and performance device can be satisfactory. Since the critical dimension in semiconductor manufacturing decreases as years go by, it becomes harder to use the conventional optical microscope to determine the overlay error between overlaid layers. Therefore, it is very important for semiconductor manufacturing to efficiently and precisely measure the overlay error between two overlaid patterned layers.
Currently, overlay measurement is applied in determining optically determinable patterns that are printed on a semiconductor wafer during manufacturing. These patterns are in a bar-in-bar or box-in-box format. Conventionally, the patterns are magnified or digitalized to quantify the overlay error using image analysis algorithm to obtain the dislocation between two overlaid layers. U.S. Pat. No. 7,160,657 discloses the bar-in-bar and box-in-box related technologies.
In 1988, Chappelow et al. provides another method for overlay measurement, wherein linear grating patterns on each of the top and bottom layers are used. The linear grating patterns on the top and bottom layers have the same pitch. The point light source is much larger than the line width of the grating patterns. In 2001, Bischoff uses the diffraction efficiency difference of the ±1st order diffractions to measure the overlay error. The pattern used by Bischoff is obtained by overlaying the linear grating patterns with the same pitch on the two overlaid layers. Identical amplitudes for the ±1st order diffractions occur due to symmetry when the center of one grating pattern is overlapped with the center of another grating pattern. The symmetry breaks if there is any dislocation between the grating patterns. The amplitude difference of the ±1st order diffractions is related to the dislocation.
Recently, H. T. Huang, adopts similar grating structure as a target to perform zero-order diffraction for measurement. A wide-band light source is used to compare the diffraction efficiency resulting from zero-order diffraction to obtain overlay data. The aforementioned optical measurement technique based on diffraction analysis on a periodic pattern is diffraction measurement. The apparatus for diffraction measurement is referred to as a scatterometer. Better resolution can be achieved using diffraction characteristics of a grating pattern than general patterns. However, the aforementioned measurement takes a longer time than the use of an optical microscope. Therefore, a theoretical model is required to compare the measured data, as disclosed in U.S. Patent Pub. No. 20060197951.